The health of an individual can be monitored using various physiological parameters. For example, in a clinical environment a physician may diagnose a particular condition based on increases or decreases in blood pressure over an extended time period. Various analytical methods have been proposed for extracting information of diagnostic value from blood pressure (BP) signals. A conventional approach for obtaining blood pressure readings for long time periods is to record the blood pressure for a 5-minute period, for example at 1-hour intervals, and assume that the blood pressure data collected during this period is representative of the blood pressure throughout the remainder of a longer time interval, during which data was not collected.
Conventional mathematical methods for analysis of a cardiovascular signal such as BP, Electrocardiography (ECG), photoplethysmography (PPG) etc. generally focus on an analysis of the beat-to-beat intervals, expressed as the heart rate variability (HRV). This time series of interval lengths may be analysed using a variety of measures in the time domain. Alternatively, the time series can be analysed in the frequency-domain (Fourier transformed power spectral density; PSD). With a long time window, the results are affected by the non-stationarity of the signal or with a short window, where the signal is more stationary, the frequency resolution is poor. Other methods derive from properties of a chaotic system such as Poincare plots, the largest Lyapunov exponent, entropy, correlation dimension, etc. While these conventional methods are able to provide some diagnostic information from the signal, the biggest drawback is that the majority of the data, namely the shape of the waveform between the intervals, is disregarded. It is known that various conditions affect the shape of the waveform, and clearly HRV ignores such changes.
Instead of using mathematical analysis of the signal to extract information, an alternative approach is to simply display the blood pressure trace, which can be viewed by a physician in order to support a diagnosis. However, information is lost when the blood pressure data is displayed for a relatively long time period, such as 1 hour or more, since display constraints mean that the data is necessarily compressed along the time axis. In blood pressure traces spanning long time periods, it is generally only possible to discern the vertical movement in the signal, and information about both the beat-to-beat shape of the waveform and the majority of the frequency-domain is lost.
The invention is made in this context.